The drugs makes no sense in the first place; there's no "turn a people into a tyrant" drug (also it's probably meant in the sense of "substance that has an effect on the body", not in the sense of "addictive substance") and even in a fantasy setting that's one heck of a convenient plot point ... no food taster, doctors, anything either? Her super-competent pseudo-harem couldn't do anything? etcetc.
Also, yeah, your average normal everyday woman just three days into her new life executing a whole bunch of people and (apparently) barely struggling with the decision, that totally makes sense as well. ^^